How do you get it working on chromecast?

I presume, by using screen mirroring: https://support.google.com/chromecast/answer/6059461

I hit the chromecast icon in the upper-right corner of my chrome browser while streaming on my laptop.

They said it plays on three of those devices which probably means the laptop, fire TV and Roku

No, it plays on all 4 devices. I said three of them played on my TV. Those three were the chromecast, fire tv and roku. Although I guess the laptop could display on my tv with an hdmi cable, so all 4 methods should work.